(1) Field of the Invention
The present invention relates to gaskets for providing a seal between machine parts and, particularly, to an improved head gasket for use between the block and cylinder head of an internal combustion engine. More specifically, this invention is directed to a novel metallic cylinder head gasket and, especially, to a multi-layer gasket having at least a first cover plate which includes a resilient primary seal defining bead and a cooperating support plate which includes a deformable structure for limiting deformation of the bead. Accordingly, the general objects of the present invention are to provide novel and improved articles of such character.
(2) Description of the Prior Art
Published European Patent No. EP 0 306 766 and U.S. Pat. Nos. 4,726,315 and 4,799,695 disclose prior art metallic cylinder head gaskets for use in the environment of an internal combustion engine. These prior art cylinder head gaskets comprise a support or carrier plate which cooperates with at least one cover plate, the cover plate having resilient beads, i.e., smoothly curved deformations, which function as sealing rings. As is well known, during the operation of an internal combustion engine, the width of the gap between the cylinder head and engine block, particularly in the regions adjacent the cylinders, varies because the operating environment is characterized by high temperature and by periodic pressure fluctuations. The cylinder head gasket, which must seal this variable gap, is thus subject to continuous changes in pressure and must have permanent resilient properties in order to maintain a satisfactory seal.
In prior art cylinder head gaskets as exemplified by the above referenced publications, a multiplicity of ring-type seals are defined by upward longitudinal extensions of the gasket, such gasket extensions defining, and thus being disposed about, extensions of the combustion chambers. These gasket extension are in the form of "beads", i.e., curved projections which seal on their convex side. The beads function as resilient seals which flex to follow vertical movements of the cylinder head relative to the engine block. For such gaskets to perform properly, the resilient beads must be prevented from undergoing unacceptably large deformation under maximum loading. Overloading of a bead may cause a rupture or permanent deformation which would deleteriously affect the sealing properties. On the other hand, the construction of the gasket must be such that the applied clamping force will not be completely relieved, i.e., the beads will remain subject to some deformation under the condition of minimum loading. The working range of the resilient beads thus lies between two extremes of deformation.
To summarize the above, in order to ensure proper functioning, the resilient beads of a metallic cylinder head gasket must not be completely deformed either during installation of the gasket, i.e., bolting of the head to the block, or under operating conditions. An unacceptably large vertical deformation of a bead, i.e., a compression which will cause damage, in respect to the nominal plane of the gasket is, in known gaskets, prevented by means of a "deformation limiter" of constant thickness. Such deformation limiters are customarily provided about the periphery of the combustion chamber extensions defined by the gasket and serve to extend the gasket upwardly about the periphery of the combustion space.
Metallic cylinder head gaskets which have previously been available have been found to be somewhat lacking in performance in the environment of recently introduced high power engines of weight-saving design. The deficiencies of prior art cylinder head gaskets in the environment of these new engines may be attributed to the different degrees of rigidity of the engine components. These different degrees in rigidity have resulted in the forces in the regions adjacent the periphery of the combustion chambers differing significantly from those in the regions adjacent to the points where the cylinder head is bolted to the engine block. Restated, a high mechanical clamping force is exerted on the cylinder head gasket in the areas near the bolts which attach the cylinder head to the engine block, this force decreasing with increasing distance from the bolts. Accordingly, a relatively small degree of further deformation will result in the sealing beads which surround the head bolts deforming from an initial clamped condition to the condition of maximum allowable deformation as determined by a cooperating deformation limiter. However, in areas displaced from the bolts, where there are smaller stress forces, the beads can experience greater movement between their conditions of minimum and maximum deformation during engine operation, the minimum deformation being characterized by a comparatively small degree of flexure.
As a consequence of the relatively small size of the surface areas where the deformation limiters contact the cover plates, a high degree of loading may cause a depression or indentation to be formed in the cylinder head, such indentation being a mirror image of the deformation limiter. When this happens, the working range of the bead is altered and, consequently, the maximum allowable deformation of the bead may be exceeded. Should this occur, there may be fracture of the gasket cover plate in the bead area.
It should be noted that the above-briefly discussed problem is exacerbated if the engine uses materials with different coefficients of thermal expansion, a gray cast engine block in combination with a light weight alloy head for example. The use of such materials with different coefficients of thermal expansion will result in the bead additionally being stressed in the direction of the plane of the gasket.